(1) Field of the Invention
The present invention relates to a method of locally coating a base metal with a molten metal coating, and more particularly to a method of galvanizing ferrous metal strip or sheet material on one side only. The present invention is also directed to the products obtained by said method.
(2) Description of the Prior Art
The purposes of one side coating of metal strips or sheets is well known and have been described in U.S. Pat. Nos. 3,121,019 and 3,149,987. Essentially one side coating of ferrous metal sheets is desirable to have corrosion resistant properties of zinc on one side of such sheet, while the opposite side has its original uncoated surface to permit i.a. paint adherence or spot welding.
The principal object of the present invention is to provide a process by means of which only a selected portion or portions of a metal piece can be galvanized. More specifically, this invention provides a method of galvanizing one side only of sheet metal. The process of the invention can be carried out either as a batch process on individual sheets, or it can be carried out as a continuous process on a strip. In either case, the steps performed on the material are the same.
In accordance with the present invention, the metal is first cleaned so that its surface is suitable for the reception of zinc. Those portions of the metal not to be galvanized are then coated with a masking material which is dried. Thereafter, the metal is heated and dipped in zinc, and, when the zinc coating has solidified, the masking material is removed. The surface that was previously covered with masking material is then clean and ready for whatever further treatment may be required.
Several masking materials have been proposed in the past. A suitable protective masking material must, however, to match the following requirements in order to provide an effective operation: It should be inexpensive, easy to apply and well adhering to the substrate during galvanization while being easy to remove after galvanization. The masking material must furthermore resist high temperature, i.e. a minimum temperature of about 450.degree.-500.degree. C., which is the temperature of the liquid zinc and preferably a temperature of 900.degree. C. in order to protect the metal against oxidation during possible further heat treatment (e.g. annealing). The masking material should also not pollute the zinc bath and remain free of zinc in order to avoid an excessive drag-out of zinc from the galvanization. Finally the masking material and technique should allow an easy and inexpensive installation of the necessary equipment for existing galvanizing lines.